The present invention relates to beamforming techniques to be used in the near-field of an array for active sonar applications.
Modern radars and sonars having multi-element arrays to receive an incoming signal that has a high signal to noise ratio. Beamforming refers to techniques for electronically focusing and steering the beam formed by the multi-element arrays in the direction of the incoming signal so as to maximize the reception of the incoming signal.
Each element of the array receives the incoming signal. Depending on the range and direction of the source radiating the signal, the geometry of the array, and the bandwidth of the incoming signal, each element may receive the incoming signal at a different time. To compensate for this time difference in reception, each element's received signal must be aligned in time corresponding to the desired range and direction, before summing the individual received signals. Such a process ensures the received signals are coherent before summing. Such time compensation can be accomplished by delaying the received signal of each element by a proper time delay before summing.
To illustrate the importance of coherency in achieving maximum reception of the incoming signal, if N coherent equal amplitude received signals are added together, the sum is N times greater than the amplitude of one received signal. If N incoherent equal amplitude received signals are added, the sum will be less than N times the amplitude of one signal and, possibly, zero.
Not only does coherency among the received signals maximize the reception of the incoming signal, but also, because noise is by nature incoherent, the lack of coherency among received noise reduces the reception of noise. Correspondingly, the sum of N equal noise measurements is the square root of N times the one noise measurement.
Consequently, beamformers have a signal to noise gain because of the coherency of time-delayed received signals and the incoherency of noise. If the incoming signal received by N elements of an array is coherent, a signal to noise gain will increase by a factor of N divided by the square root of N.
Moreover, other advantages are achieved by time delaying the received signals to achieve coherency among them. A correlated signal as well as incoming signal may be received by the array. The time delays, however, are appropriate for achieving coherency of received signals, which may not achieve coherency for received correlated signals. Thus, an incoming signal to correlated signal gain occurs because the reception is maximized for the incoming signal but not the correlated signal. Furthermore, shading of the arrays, which reduces sidelobes that are pointing in the direction of the correlated signal or source, can improve the incoming signal to correlated signal gain, but at the expense of reducing the incoming signal to noise gain.
There are three known basic beamformer types for achieving coherency among the received signals of each element: true time delay, delay interpolate, and phase shift. A true time delay beamformer samples the received signal at a fixed rate higher than necessary for the proper time delay for beamforming. This results in an unnecessary amount of sampling. Any proper time delay may be applied by selecting a set of samples corresponding to the proper time delay out of all the samples taken. Alternatively, the sampling rate may be adjustable to achieve a set of samples for a particular time delay necessary for beamforming, hence, avoiding unnecessary samples.
Second, a delay interpolate beamformer is similar to a true time delay beamformer except received signals are sparsely sampled. Because of the sparse sampling, the appropriate set of samples may not exist that are necessary for applying the proper time delay. To remedy this, the appropriate set of samples are estimated by interpolating between the two closest sets of actual samples.
Last, a phase shift beamformer assumes that the amplitude and phase of the received signal has a constant frequency and maximum amplitude during the proper delay time. Accordingly, the received signal's amplitude and phase at the time delay is derived by phase shifting. That is, the received signals are phase-shifted by an amount equal to the proper time delay multiplied by the assumed frequency of the incoming signal.
All conventional art is simply modifications of these three basic schemes.
The selection of beamformer type for a particular application is influenced by the lowest system cost in terms of dollars, space, and/or power for a given level of performance. For conventional state-of-the-art beamformers using digital computation techniques, lowest system cost means finding the most computationally efficient method--computational efficiency being mainly determined by the number of multiplications.
For example, in the case of a small aperture array receiving a narrow band incoming signal, phase shift beamforming is an efficient method because a Fast Fourier Transform (FFT) algorithm can process the received signals with a relatively low number of multiplications.
Phase shift beamforming, however, is not adequate for large aperture arrays receiving a broadband incoming signal, an example of such is seen in FIG. 1, when the reciprocal of the incoming signal's bandwidth exceeds the proper time delays required to achieve coherence among the received signals of the elements. In these situations, mainlobe spreading and high level sidelobes occur because the amplitude and phase of one received signal of an incoming signal cannot be correlated with the amplitude and phase of another received signal of the same incoming signal.
For broadband incoming signals, the prior art has not established a satisfactory solution. True time delay beamformers are always applicable, but rarely employed because the large number of samples that must be taken and stored is too costly.
Most attempts to achieve a computationally efficient method for broadband incoming signals have involved sampling at the Nyquist rate of the system bandwidth, which is dependent on the bandwidth of the transmitted signal and received signal, or higher, and interpolators for interpolating between samples, as needed. A beamformer using this method is described in Harvat, Bird, Goulding, "True Time-Delay Beamforming," IEEE Journal of Oceanic Engineering, Vol. 17, No. 2, April 1992. This method is computationally intense compared to the efficient FFT algorithms used in the phase shift beamformer. Recognizing the inherent computational disadvantage of this method, conventional art has tried to formulate hybrid beamformers, which combine interpolator based sections and phase shift sections to partially regain some of the computational advantage of a pure phase shift beamformer, as described in Gabel, Kierth, "Hybrid Time-Delay/Phase-Shift Digital Beamforming for Uniform Collinear Arrays," J. Acoust. Soc. Am., June 1984.
The particular problem giving rise to the solution provided by the present invention is a high resolution Side Looking Sonar (SLS) array requiring time delays that exceeded the reciprocal of the incoming signal's bandwidth by a factor of two. A simple phase shift beamformer could not be used because of the short duration of the incoming signal and the large aperture of the array. As a result, the incoming signal passes beyond the center elements of the array before it arrives at the end elements of the array.